Consider making num.clamp generic?

[leafpetersen]

Currently num.clamp is defined as (num, num) -> num. Making this generic would allow better static typing.

cc @floitschG @lrhn

[leafpetersen]

See #30402 for some context.

[lrhn]

It won't be easy to do that and preserve the current behavior, or maybe doing it at all.

What signature would you suggest for the method?

T clamp<T extends num>(T low, T high);  

This won't work.
The call x.clamp(y, z) can return any of x, y or z. That means that num x = 42; x.clamp(0.0, 100.0); will likely infer T as double and complain that the return value is int.

The problem is that the actual constraint is that T must be a type that both arguments and the value itself can be assigned to. We cannot express that constraint.

Even if we specialize the method for int and double, so we basically know the type of this, we still need:

T clamp<T extends ??>(T low, T high);

where ?? somehow restricts T to int and num, but avoids double

Maybe just add

int int.clampInt(int low, int high);
double double.clampDouble(double low, double high);

?

[leafpetersen]

Hmm, yeah, unfortunate. Overloading might help here, if you could use the type of the receiver.

Another option would be:

//num
num clamp(covariant num low, covariant num high);

//int
@override
int clamp(int low, int high)

[lrhn]

Using covariant wouldn't work very well either.
In practice, it means that you have int int.clamp(int,int) and double double.clamp(double, double), but there is no way you can call the clamp of num x; without testing the type of it first, and then you could just remove the num.clamp declaration completely and not be any worse off.

I think the best solution is to remove clamp from num and put it on int and double, or to keep it on num and put clampInt and clampDouble on int and double.

Or remove it from num and make it a static function T clamp<T extends num>(T value, T min, T max). Then it can be used with both int, double and num.
(I don't see a good way to get "self" into the type system, short of adding self-types like This clamp<T extends This>(T min, T max)).

[floitschG]

We are dropping clamp from num and specialize the functions on int and double.

[lukepighetti]

Friendly ping for forward movement on this. Spent a few hours debugging an issue with num.clamp() on a double when I was expecting a double out the other side.

[lrhn]

We will allow better static typing through special casing in the type system.